1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more specifically, to a method for manufacturing a complementary metal oxide semiconductor (CMOS) image sensor.
2. Description of the Related Art
An image sensor, as a kind of semiconductor device, transforms optical images into electrical signals. Image sensors can be generally classified into charge coupled devices (CCDs) and CMOS image sensors. Among these image sensors, a CMOS image sensor comprises a photo diode for detecting incident light and transforming it into electrical signals, and logic circuits for transmitting and processing the electrical signals.
In processes for manufacturing a CMOS image sensor, it is desired to increase a so-called fill factor, defined as a ratio of light sensing area to total image sensor area, for the purpose of improving light sensitivity. However, since the light sensing part is formed only in an area other than the area where logic circuits are formed, there are limits to improving the fill factor of the device. For such reason, forming a plurality of microlenses on or over color filters of a CMOS image sensor has been widely employed as one alternative light condensing technique for changing a path of light incident on regions other than the light sensing part and concentrating light to the light sensing part.
A conventional method for manufacturing a CMOS image sensor including microlenses is hereinafter described referring to FIGS. 1a to 1d. 
A conventional CMOS image sensor comprises: a light sensing part 13 including a photo diode 11 for accepting incident light, and for generating and accumulating electric charges; a protecting layer 21 formed on a structure of the light sensing part; color filter arrays 23; a planarization layer 25; and a plurality of microlenses 27.
In a conventional method of manufacturing such structured CMOS image sensor, as shown in FIG. 1a, the protecting layer 21 with a silicon nitride base is formed on a semiconductor substrate 10 that comprises the light sensing part 13 including the photo diodes 11, and on a wiring bonding pad 15. Then, as shown in FIG. 1B, a portion of the protecting layer 21 on the wiring bonding pad 15 is removed, exposing an upper surface of the wiring bonding pad 15. This opening process generally involves a photolithography process. More specifically, a photoresist material is applied and patterned on the protecting layer 21, then a portion of the protecting layer 21 is etched and removed to expose the wiring bonding pad 15. Afterwards, a remaining photoresist material is removed using a reactive ion etch.
Next, as shown in FIG. 1b, the color filter array 23 is formed on the protecting layer 21. Here, the color filter array 23 is formed in a primary color system, i.e., including a red filter (R), a green filter (G), and a blue filter (B), using photoresist materials containing a red, green, or blue pigment, respectively. Formation of each color filter involves a series of coating, exposure and development processes according to the photolithography technique. Alternatively, the color filter array can be formed in a complementary color system including cyan, yellow, and magenta filters.
Then, as shown in FIG. 1c, the planarization layer 25 is formed on the color filter array 23. The planarization layer 25 removes steps (uneven horizontal surfaces) in the topography between the color filters 23, thus enabling uniform formation of microlenses. In addition, a thickness of the planarization layer 25 is controlled so that a focal length is adjusted appropriately. The planarization layer 25 can comprise a photoresist, oxide, or nitride base material.
Next, a photoresist layer is applied, exposed, and developed on the planarization layer 25, thus forming a plurality of photoresist patterns. These photoresist patterns are thermally reflowed and cured to form a lens, thus resulting in a plurality of microlenses 27 shown in FIG. 1d. 
According to the above-described conventional method, the remaining photoresist material in the bond pad opening process is ashed by a reactive ion etch. Thus, surface properties of the protecting layer 21 can be locally changed according to the conditions (ambient and other) in the ashing process. As a result, adhesion of color photoresist on the protecting layer 21 may deteriorate so that some of the patterned color filters peel off. In general, the color filter array 23 is formed of organic materials. Especially, in the case where the color photoresist contains a large amount of pigment, the peeling phenomenon may occur more frequently because of relatively large-sized pigment particles affecting adhesion to the protecting layer 21. There is a need in the art to solve the peeling problems of the color filters because it can induce deterioration and/or failure of certain characteristics of the device (e.g., discoloration).
In the case where the peeling phenomenon occurs, the color filter array 23 and microlenses 27 may be reworked and reproduced by stripping one or more of the color filter array 23, the planarization layer 25, and the microlenses 27, and repeating the photolithography process(es) for the color filter array 23, the planarization layer 25, and the microlenses 27, up to several times. However, a developing solution used in the photolithography process generally comprises TMAH ((Tetramethylammonium hydroxide) which erodes the exposed wiring bonding pad 15. Therefore, the number of repetitions of the stripping and photolithography process is restricted. Moreover, several repetitions of the photolithography process can lead to contamination of wiring bonding pad 15, thus resulting in a wiring failure.
Meanwhile, according to the conventional method, the microlenses 27 are formed distant from each other by about 0.2 μm˜0.5 μm, for the purpose of preventing formation of bridges between the microlenses 27 during the curing and reflowing processes of the corresponding photoresist pattern. However, the gap between microlenses 27 results in at least some loss of the light incident between microlenses 27, and especially a problem that the resolution of color signals may be less than optimal due to oblique light incident to adjacent pixels.